1. Field of the Invention
The present invention relates to a developing device and a developing method for use in an image forming apparatus such as a copier, a laser printer, a facsimile apparatus and the like, for forming an image in an electrophotographic system; and in particular to a developing device and a developing method for visualizing an electronic latent image such as a latent charge image and an electrostatic latent image formed on a surface of an image data forming member by use of a developer.
2. Description of the Related Art
In this specification, the term "develop" means visualizing an electronic latent image formed on a surface of an image data forming member by making a developer formed of electrically charged colored particles adhere to such an electronic latent image. Examples of the electronic latent image are a conductive latent image, a latent charge image on photoconductive particles and an electrostatic latent image. Such a developing system will be described using as an example of an electrostatic latent image formed on a photoconductive member having a photoconductive layer thereon.
Developing mainly includes three steps: (1) electrically charging toner particles, (2) transporting the toner particles to a developing region, and (3) making the toner particles adhere to an electrostatic latent image. A practical developing process depends on the type of the developer. The electrophotographic system is mainly classified into a dry developing system and a wet developing system. The dry developing system is further classified into two systems: one system in which a two-component developer is used containing a toner formed of a resin mixed with a colorant and a carrier formed of magnetic iron powders and the like; and another system in which a monocomponent developer formed only of a toner is used.
In the system using a two-component developer, particles of the toner and particles of the carrier are first stirred in a developer chamber, thereby charging the toner and the carrier with opposite polarities to each other by friction. The toner particles which adhere to surfaces of the carrier particles are transported to a developing region by a developer retaining member (for example, a developer roller). The system using a two-component toner is further classified into several different systems depending on the carrier for transporting the toner: a magnetic brush development system using a carrier formed of iron powders, a cascade development system using a carrier formed of a glass material or a nonferrous metal, and the like.
In the system using a monocomponent developer, a blade is provided so as to be opposed to the developer retaining member. The toner is charged by friction with the blade while being transported by the developer retaining member toward a developing region. The system using a monocomponent toner is further classified into several different systems: a powder cloud development system, a contact development system, a jumping development system, a magnedynamic process and the like. In the powder cloud development system, the toner particles are used in an aerosol state. In the contact development, the toner particles are directly contacted on an electrostatic latent image. In the jumping development system, the toner particles are electrically charged and jump across an air gap toward an electrostatic latent image with no direct contact. In the magnedynamic process, a magnetic, conductive toner is contacted on an electrostatic latent image.
In a system using a monocomponent toner, step (1) of electrically charging the toner has an inconvenience in that the toner particles are not sufficiently charged in a short period of time by only the friction among toner particles. In order to compensate for this inconvenience, a regulating member is mechanically put into direct contact with a roller-like developer retaining member so as to charge the toner particles by friction between the developer retaining member and the regulating member, among the toner particles, and between the regulating member and the toner particles. Such a process involves the following problems.
(a) The grain size of the toner particles is changed by the contact between the toner particles and the regulating member.
(b) The toner particles flocculate by the contact with the regulating member.
For step (2) of transporting the toner particles to a developing region and step (3) of making the toner particles adhere to an electrostatic latent image, the following system is known. A developing retaining member is directly contacted with the photoconductive member so as to adhere the toner particles on the electrostatic latent image (contact development system for a monocomponent developer). According to this system, there is a problem in that the toner particles easily adhere to an area of the surface of the photoconductive member where no image is to be formed.
The jumping development system which is a practical example of a non-contact development system for a monocomponent developer is described in Japanese Patent Publication No. 58-32375. According to this system, the toner particles are electrically charged and jump across an air gap toward an electrostatic latent image with no direct contact as mentioned above. The above problem is solved by this system although only a magnetic toner can be used. The jumping development system has, for example, the following features: (a) the toner particles are uniformly charged by use of a regulating member formed of a magnetic material; and (b) the thickness of a toner layer is reduced by the magnetic force applied on the developer retaining member.
The Jumping development system has a problem in that it is difficult to color the toner particles cyan, magenta, yellow and the like, since a magnetic toner must be used. Accordingly, it is difficult to use a developing device for this system with a color electrophotographic apparatus.
In a system using a two-component developer, a developer containing toner particles and carrier particles in a mixed state is used, and the toner particles are consumed in a larger amount than the carrier particles. The difference in the amount consumed between the toner particles and the carrier particles is increased with the progress of the developing process. Accordingly, this system has the following defects: (a) the image density fluctuates in accordance with the change in the mixture ratio of the toner particles and the carrier particles along the time passage; and (b) image quality is declined by the deterioration in the quality of the carrier particles. These defects cause the following problems.
(1) Maintenance for exchanging the developer is necessary.
(2) Since the toner particles are contacted on an area of the photoconductive member where no image is to be formed by the developer retaining member as in a monocomponent contact developing system, the toner particles easily adhere to such an area.
In a system using a monocomponent developer, the above-mentioned change in the grain size and the flocculation cause fluctuation in the amount of electricity of the charged toner particles and also blur an area of the photoconductive member where no image is to be formed. Further, in this system, the toner is charged by a friction method using friction between the toner particles and a charging member (for example, a blade formed of a resin), a charge injection method by which charges are directly supplied from an electrode to the toner particles, or the like. In order to sufficiently charge the toner particles, it is necessary to reduce the thickness of the toner layer. However, the reduction in the thickness of the toner layer reduces the amount of toner supplied to the photoconductive member, resulting in difficulties in obtaining a sufficient image density.
Recently, use of toner particles having a smaller grain size than that of conventional toner particles has been proposed in order to enhance the resolution of an image formed with toner. However, as is described in "Effects of Small-grain-size Toner on Image Quality", by Naomitsu Yoshimura, Electrophotography Japan, Vol. 31, No. 1 (1992), pp. 82-86, the amount of electricity of the charged toner particles per unit mass, namely, the toner charge-to-mass ratio (.mu.C/g) is increased in the case of the toner particles having the smaller grain size. Accordingly, in a system using a monocomponent developer, toner particles are repelled by each other, thereby reducing the toner density of the toner layer. This phenomenon causes difficulties in obtaining a sufficient image density. In order to enhance the image density, it is necessary to increase the grain size of the toner particles or the amount of the toner. An increase in the grain size will counteract the use of the toner particles having the smaller grain size. A simple increase in the amount of the toner supplied to the developer retaining member will raise the thickness of the toner layer, and thus cause toner dropping or blur on an image. These phenomena decrease the resolution.
In a system using a two-component developer, the toner particles and the carrier particles are sufficiently stirred in a developer chamber. Thus, the toner particles are sufficiently and uniformly charged. Moreover, since the toner is transported on the developer retaining member after being charged, a sufficient amount of toner is transported so as to obtain an image with a high density. However, in this system, the developer contains toner particles which are not sufficiently charged because the amount of electricity of the charged toner is changed along the time passage by toner consumption. Accordingly, there is an undesirable possibility of reduction and non-uniformity in the image density. In order to maintain the image density at a constant level, the mixture ratio of the toner particles and the carrier particles in the developer is required to be kept constant. This necessitates a complex structure of the entire developing device.
In the case where the developing system using a monocomponent developer formed of a non-magnetic toner is used in a color electrophotographic apparatus, the toner particles are not sufficiently charged in a short period of time only by the friction among the toner particles. In order to compensate for this inconvenience, the ratio in the circumferential velocity of the developer retaining member such as a developer roller with respect to that of the photoconductive member is required to be increased so as to frequently cause friction. Such an increase in the above ratio involves a defect of possibly lowering the image density.
One of conceivable reasons of the above-mentioned problems is that there exist ill-charged toner particles among well-charged toner particles on the developer retaining member for some reason, and both of these toner particles adhere directly to an area of the photoconductive member from the developer retaining member.